1. Field of The lnvention
The invention relates to IR imaging systems and in particular to a wide angle IR imaging system in which at least a portion of the optical system must be maintained in a cryogenic environment.
2. Prior Art
Wide angle optical systems impose difficult conditions for infrared imaging systems. A combination of a short focal length lens requirement and the need for effective cold shielding force at least one lens element to be fully within the cryogenic environment, where it is difficult to adjust the focus. In addition, assuming that one short focal length lens element is within the cryogenic environment, the design of an optimum aperture for admitting the focused IR image field to the maximum and screening out the unfocused IR background to the maximum becomes important. Typically with single short focus element lenses the loss in lighting efficiency at the margins of the aperture can be as high as 35%. In addition, the sizes of apertures displaced from the center of the lens group are always greater than the apertures of the lenses themselves, further reducing the cold aperture efficiency.